The bayonet connection is a close tolerance fit between male and female, forming a thin cylindrical cavity. During cryogenic transfer, gas from the process liquid is vaporized in the long narrow cavity forming a vapour seal, which is at process stream temperature at one end and ambient temperature at the other. The vapour seal isolates the mechanical seal from the cryogenic temperatures. This unique feature minimizes heat - leak, reduces frosting and allows for rapid disconnection of the line. It provides an economic method of joining vacuum-insulated transfer lines. They allow for liquid savings because of low heat influx, decreased installation time, and lower field installation costs.